Desorption ion sources, especially ion sources for the ionization of samples by matrix-assisted laser desorption (MALDI), are increasingly being used for the ionization of large molecules, for example large biomolecules or artificial polymers. Ever increasing sample throughput is required.
In MALDI ion sources, bombardment with a pulse of laser light generates a plasma cloud each time, from which the ions formed are then extracted by means of an accelerating field. The plasma cloud also partially contains solid or liquid spray particles from the quasi-explosion of the matrix material. The plasma cloud expands further, depositing part of the material, matrix substance and analyte substance vaporized or sprayed in this way on the accelerating electrodes, mainly on the first acceleration electrode. As an alternative to the accelerating electrode, this type of desorption ion source can also incorporate a set of guide electrodes. After a few hundred thousand shots, there is a visible coating on these electrodes. This insulating coating can become charged and thus lead to interference of the acceleration process. The coating must therefore be removed.
The only method known until now for removing this coating is manual cleaning after venting and opening the ion source. The cleaning is usually carried out using solvents such as ethanol or acetone, and can usually be done without removing the accelerating electrode. But even without disassembling the ion source, cleaning, including the restoration of a good vacuum, takes a few hours and often requires a new adjustment and usually a completely new calibration after the mass spectrometer has been restarted.
In the following, the accelerating electrodes and the set of ion guide electrodes which are present in a desorption ion source in analytic operation (i.e. not during the cleaning operation) opposite the sample support plate are referred to collectively as “ion guide electrodes”.
A method of cleaning, in particular, of the first accelerating electrode without opening the ion source is indispensable for genuinely high throughput operation; accelerating electrodes which are usually further away remain uncontaminated for a longer time. For sustained operation, however, it is also necessary to clean the more distant accelerating electrodes.
The ion source usually also contains a video camera and a spot light to identify the samples on the carriers.